There is an increasing demand for a high power factor (PF) (e.g., PF>0.9) and a low total harmonic distortion (THD) (e.g., THD<20 percent (%)) over wide output power ranges across universal line voltages (e.g., 90 to 277 volts alternating current (Vac)). It is well known in the art that the input electromagnetic interference (EMI) capacitors in a single-stage power converter play a negative role to PF and THD. The contribution from the EMI capacitors becomes significant when operating with a high line voltage and a low output power, and prevents the system from achieving satisfactory PF and THD.
Currently, there are two ways to achieve high PF and low THD in a single-stage power converter. One method is to use a closed-loop control to derive an on-time value for the power transistor, and apply the on-time value across the whole line period to realize a constant on-time control to achieve a high power factor. The second method is to sample the input voltage, and a closed-loop determines a multiplier (i.e. a constant). The multiplication of the sensed input voltage and the multiplier results in a current envelope that has the same phase as the input voltage. This peak current threshold envelope defines the power transistor peak current value. Further, this peak current threshold envelope follows the input voltage waveform. If the input voltage has little distortion on the alternating current (AC) input mains, this method results in high PF and low THD. Unfortunately, when operating at low output power levels, and especially at high line voltages, the EMI capacitors create signal distortion, thereby increasing the THD and decreasing the PF.
Historically, for single-stage power converters, PF and THD specifications focused on high load operating conditions, and the PF and THD specifications were less stringent at low load operating conditions. However, there is currently a desire to impose stricter PF and THD specifications for low load operating conditions, which is difficult for existing solutions to achieve.
As such, there is a need for an improved single-stage power converter design that provides high PF and low THD when operating with a high line voltage and a low output power.